1. Field of the Invention
This invention relates to solid state lighting (SSL) and in particular to SSL luminaires having a plurality of LED chips or LED packages whose emission combines to produce light with the desired characteristics.
2. Description of the Related Art
Light emitting diodes (LED or LEDs) are solid state devices that convert electric energy to light, and generally comprise one or more active layers of semiconductor material sandwiched between oppositely doped layers. When a bias is applied across the doped layers, holes and electrons are injected into the active layer where they recombine to generate light. Light is emitted from the active layer and from all surfaces of the LED.
In order to use an LED chip in a circuit or other like arrangement, it is known to enclose an LED chip in a package to provide environmental and/or mechanical protection, color selection, light focusing and the like. An LED package also includes electrical leads, contacts or traces for electrically connecting the LED package to an external circuit. In a typical LED package 10 illustrated in FIG. 1, a single LED chip 12 is mounted on a reflective cup 13 by means of a solder bond or conductive epoxy. One or more wire bonds 11 connect the ohmic contacts of the LED chip 12 to leads 15A and/or 15B, which may be attached to or integral with the reflective cup 13. The reflective cup may be filled with an encapsulant material 16 which may contain a wavelength conversion material such as a phosphor. Light emitted by the LED at a first wavelength may be absorbed by the phosphor, which may responsively emit light at a second wavelength. The entire assembly is then encapsulated in a clear protective resin 14, which may be molded in the shape of a lens to collimate the light emitted from the LED chip 12. While the reflective cup 13 may direct light in an upward direction, optical losses may occur when the light is reflected (i.e. some light may be absorbed by the reflector cup due to the less than 100% reflectivity of practical reflector surfaces). In addition, heat retention may be an issue for a package such as the package 10 shown in FIG. 1, since it may be difficult to extract heat through the leads 15A, 15B.
A conventional LED package 20 illustrated in FIG. 2 may be more suited for high power operations which may generate more heat. In the LED package 20, one or more LED chips 22 are mounted onto a carrier such as a printed circuit board (PCB) carrier, substrate or submount 23. A metal reflector 24 mounted on the submount 23 surrounds the LED chip(s) 22 and reflects light emitted by the LED chips 22 away from the package 20. The reflector 24 also provides mechanical protection to the LED chips 22. One or more wirebond connections 11 are made between ohmic contacts on the LED chips 22 and electrical traces 25A, 25B on the submount 23. The mounted LED chips 22 are then covered with an encapsulant 26, which may provide environmental and mechanical protection to the chips while also acting as a lens. The metal reflector 24 is typically attached to the carrier by means of a solder or epoxy bond.
LEDs and LED packages, such as those shown in FIGS. 1 and 2, are more commonly being used for lighting applications that were previously the domain of incandescent or fluorescent lighting. The LEDs and LED packages can be arranged as the light source in SSL luminaries or lamps and single or multiple LEDs or LED packages can be used. The general acceptance of these luminaries has accelerated with the improvement in LED emission efficiency and quality. LEDs have been demonstrated that can produce white light with an efficiency of greater than 150 L/W, and LEDs are expected to be the predominant commercially utilized lighting devices within the next decade.
The light generated by different light sources can be measured in terms of color rendering index (CRI or CRI Ra) and color temperature. CRI is a quantitative measurement of the ability of a light source to reproduce the colors of various objects faithfully in comparison with an ideal or natural light source. Light sources with a high CRI approaching 100 can be desirable in color-critical applications such as photography and cinematography. Daylight has a high CRI of approximately 100 and incandescent bulbs have a relatively close CRI of greater than 95. By comparison, fluorescent lighting has a lower CRI in the range of 70-80, and mercury vapor or sodium lamps have a much lower CRI of 40 or less. High quality light suitable for general indoor illumination should have a CRI of greater than 90.
Color temperature is a characteristic of light source that is determined by comparing the light's chromaticity with that of an ideal black-body radiator. The temperature (usually measured in kelvins (K)) at which the heated black-body radiator matches the color produced by the light source is that source's color temperature. For incandescent light sources the light is of thermal origin and is very close to that of an ideal black-body radiator. Higher color temperatures of 5000 K or more are “cool” and have green to blue colors while lower color temperatures of 2700 to 3500 K are considered “warm” and have yellow to red colors. General illumination can have a color temperature between 2,000 and 10,000 K, with the majority of general lighting devices being between 2,700 and 6,500 K.
In contrast to incandescent radiation, light sources, such as fluorescent lamps emit light primarily by processes other than raising the temperature of a body. This means the emitted radiation does not follow the form of a black-body spectrum and these sources are assigned what is known as a correlated color temperature (CCT). CCT is the color temperature of a black body radiator which to human color perception most closely matches the light from the lamp. For high quality light sources it is also important that color of the illumination be as close as possible to that of a black body spectrum (i.e. black body locus on CIE chromaticity diagram). On such standard deviation is measured in terms of a MacAdam ellipses with a suitable proximity to the black body locus typically being within a 4-step MacAdam ellipse.
SSL luminaires have been developed that utilize a plurality of LED chips or LED packages, with at least some being coated by a conversion material so that the combination of all the LED chips or packages produces the desired wavelength of white light. Some of these include blue emitting LEDs covered by a conversion material such as YAG:CE or Bose, and blue or UV LEDs covered by RGB phosphors. These have resulted in luminaires with generally good efficacy, but only medium CRI. These have not been able to demonstrate both the desirable high CRI and high efficacy, especially with color temperatures between 2700 K and 4000 K.
Techniques for generating white light from a plurality of discrete light sources to provide improved CRI at the desired color temperature have been developed that utilize different hues from different discrete light sources. Such techniques are described in U.S. Pat. No. 7,213,940, entitled “Lighting Device and Lighting Method”. In one such arrangement a 452 nm peak blue InGaN LEDs were coated with a yellow conversion material, such as a YAG:Ce phosphor, to provide a color that was distinctly yellow and had a color point that fell well above the black body locus. This yellow emission can comprise blue light component that leaks through the yellow conversion material. The yellow emission is combined with the light from reddish AlInGaP LEDs that “pulls” the yellow color of the yellow LEDs to the black body curve to produce warm white light. FIG. 3 shows a CIE diagram 30 with the tie lines 32 between red light 34 from red emitting LEDs and various yellow and yellowish points on the blue/YAG tie line 36. With this approach, high efficacy warm white light with improved CRI can be generated. Some embodiments exhibited improved efficacy, with CRI Ra of greater than 90 at color temperatures below 3500 K.
This technique for generating warm white light generally comprises mixing blue, yellow and red photons (or lighting components) to reach color temperature of below 3500 K. The blue and yellow photons can be provided by a blue emitting LED covered by a yellow phosphor. The yellow photons are produced by the yellow phosphor absorbing some of the blue light and re-emitting yellow light, and the blue photons are provided by a portion of the blue light from the LED passing through the phosphor without being absorbed. The red photons are typically provided by red emitting LEDs, including reddish AlInGaP LEDs. Red LEDs from these materials can be temperature sensitive such that they can exhibit significant color shift and efficiency loss with increased temperature. This can result in luminaires using these LEDs emitting different colors of light different temperatures. To compensate for these variations, luminaires can include compensation circuitry that can vary the drive signal applied to the LEDs. This, however, can increase the cost and complexity of the luminaires. To avoid these costs and complexities, the red light can be provided by LEDs that are covered by a red phosphor that absorbs substantially all of the LED light and re-emits red light. The efficiency of these emitters, however, is limited by the relatively low conversion efficiency of red phosphors. This low conversion material efficiency can reduce the emission efficiency of the luminaire.